Comet C/2023 A3 (Tsuchinshan–ATLAS)
C/2023 A3 (Tsuchinshan-ATLAS) - Click here for full resolution
Comet C/2023 A3 (Tsuchinshan–ATLAS) is a comet from the Oort cloud discovered by the Purple Mountain Observatory in China on 9 January 2023 and independently found by ATLAS South Africa on 22 February 2023. The comet passed perihelion at a distance of 0.39 AU on 27 September 2024, when it became visible to the naked eye. Tsuchinshan-Atlas peaked its brightest magnitude shortly after passing the Sun at 9 October, with a magnitude of −4.9 per reported observations at the Comet Observation Database (COBS).
Source: Wikipedia
NGC/IC:
Other Names:
Object:
Constellation:
R.A.:
Dec:
Transit date:
Transit Alt:
n.a.
C/2023 A3 (Tsuchinshan-ATLAS))
Comet
Cancer
14h 53m 08s
+00º 19.7’
n.a.
n.a.
Conditions
The chance to see this comet from the northern hemisphere was brief. Until it reached its closest point to the sun on 27 September 2024, it was mainly visible from the southern hemisphere. After that, it was too near the sun to be seen. In mid-October, the comet could be seen in the western sky at dawn, but it was quickly losing brightness. On 14 October, the sky was clear, and the comet was visible at 20:00h, half an hour after sunset, at an altitude of 8° in the west. Photos were taken around that time, but about 15 minutes later, low clouds and some mist obscured the comet.
Equipment
Images on Astrobin revealed that the tail of the comet was quite large, so the best changes of capturing the object as a whole was with a regular photo camera in combination with a short telelens. For this purpose the Leica SL3 camera was used, which is a full-frame mirrorless camera. The Leica APO-Summicron-SL 90mm f/2.0 was used as the objective. The fase f/2.0 aperture would allow for very short exposure times, so the camera was mounted on a stationary tripod, without tracking device.
Camera/Lens
Mount
Filters
Guiding
Software
Leica SL3, Leica APO-Summicron-SL 90mm f/2.0
Arca-Swiss Cube tripod head, Gitzo GT3543XLS tripod
n.a.
n.a.
PixInsight 1.8.9-3, Capture One 2024, Affinity Photo 2.5
Imaging
The comet was very well visible with the naked eye! And while less bright than expected, its enormous tail was very impressive. The imaging started just a few minutes before 20:00h. Short bursts of 10 frames were shot at slightly different exposure times and ISO settings. It turned out that even at a few seconds, stars were still a bit elongated due to the non-tracking setup. Therefore the final set of 10 images that was chosen was shot at an exposure time of 1.6s at ISO800. The comet was a bit too high in the sky to capture the full comet as well as the horizon in the same shot. Therefore a separate shot of the horizon was shot that was later blended in.
Resolution
Focal length
Pixel size
Resolution
Field of View
Rotation
Image center
8036 × 5773 px (46.4 MP)
90 mm @ f/2.0
3.76 µm
8.74 arcsec/px
21º 24’ x 15º 17’
36.5 degrees
RA: 15h 13m 58.715s
Dec: +01° 26’ 14.68”
Processing
All ten images were calibrated using 10 matching darks, registered and stacked using the WeightedBatchPreProcessing (WBPP) script in PixInsight. The timescale within which these images were collected was so short (total less than 30s), that no specific precautions were made for any movement of the comet relative to the stars within this timespan. So the regularly stacked image from WBPP was used for further processing.
With the images taken so short after sunset, and with the moon already up, the image had a major gradient that needed to be corrected. All available background extraction methods were tried. Best results were obtained by a first run with GraXPert, followed by DynamicBackgroundExtraction (DBE) with a very fine point-grid. BlurXterminator was applied in ‘Correct Only’ mode, to just correct some of the star elongation. SPCC was used for colour calibration. SPCC needs a plate-solved image to work with, and obviously the image, coming from a regular photo camera, did not have any Fits header to be of help for that. So the Blind Solver 2000 script of SetiAstro was used, which did a great job in getting basic coordinates, followed by the PlateSolver process to obtain exact image location and dimensions.
In the STF-stretched version of the image, the anti tail of this comet was clearly visible. But at that stretch-level, there was a lot of left-over background gradients and noise visible. What followed was a careful stretching of the image, trying to separate the anti tail from its background. This was somewhat successful. After the final stretch some noise reduction was applied using NoiseXTerminator. Also this needed to be done carefully, to not reduce the anti tail too much. Some of the final blotchy parts of the background were removed by reducing the colour saturation while selecting only background using a range-mask.
The image of the comet while it was still a bit higher on the horizon. The focus was here to extract the comet-signal from its background, which was very hard. The huge gradients in combination with the very low brightness of the anti tail relative to its background required some careful stretching and curves manipulations.
As a final step, the isolated comet image was combined with an image of the horizon. This was done by blending the two layers in Affinity Photo, using the colour dodge blending. Stars from the horizon file needed to be removed, which was a bit of a tedious task using the inpainting brush. Alternatively I could have masked out just the horizon, but since the comet image was created as a completely black background image, I needed the blueish background of the sky from the horizon image.
This all means that the final image is technically not exactly as how the comet was seen. The comet is placed a little bit lower on the horizon, where it was standing 10-15 minutes after the actual comet images were taken.
This image has been published on Astrobin.
